This invention relates to a modified Karl Fischer reagent for determining water, which contains a pyridine substitute, sulfur dioxide and iodine, and to a method for determining water by means of this reagent.
The literature contains a number of proposals for replacing pyridine in the Karl Fischer reagent by other substances. In Anal. Chim. Acta 94, 395 (1977) the pyridine is replaced by sodium acetate. However, this replacement has certain disadvantages involved. Acetate esters are formed, for example with the alcohol used as the solvent, the reaction setting free water, which is of course an interferant in the method for determining water. The solutions are unstable as a result, their blank value increasing steadily.
British Pat. No. 728,947, aside from acetates, also mentions alcoholates, phenolates and metal salts of weak organic acids as replacements for pyridine. On testing the substances named in this patent, it was found that they are unsuitable replacements for pyridine, partly owing to insufficient solubility, partly owing to insufficient stability of the completed solutions. It does not take long for precipitates to form or the titer of the solution to fall drastically.
To avoid these disadvantages, a very recent proposal (European Pat. No. 35, 066) is to replace pyridine by aliphatic amines in a certain molar ratio to sulfur dioxide or by heterocyclic compounds. However, even this replacement for pyridine still has disadvantages due to the fact that the stability of the end point varies with the amount of water to be titrated. It is true that using these reagents is a way of keeping the unpleasant pyridine odor away from the user, but most of the nitrogen compounds used as replacements for pyridine are still toxic.